1. Field of the Invention
The present invention relates to an industrial polyester filamentary yarn and a tire cord formed from this filamentary yarn. More particulary, it relates to a polyester filamentary yarn, a tire cord having an improved fatigue resistance due to increased thermal dimensional stability and strength, and low shrinkage as well as a process for production of this yarn.
2. Description of the Prior Art
In general, nylon, rayon, polyester etc. fibers are typical examples of the fibers which have been used as reinforcements in rubber tires. Nylon tire cord has higher strength and toughness than the other materials due to the inherent properties of nylon fiber and has been generally used in bias tires for trucks, buses etc. Rayon tire cord provides a low degree of shrinkage and has good thermal and dimensional stability due to the inherent properties of rayon fiber and has been generally used in high speed radial travelling tires for passenger cars.
However, nylon tire cord has poor dimensional stability due to its low modulus characteristics and high shrinkage and further exhibits flat spots due to its low glass transition temperature(T.sub.g). Rayon tire cord also has low modulus characteristics and exhibits a sharp decrease in strength after the fibers have been formed into a tire cord.
In view of these defects found in both nylon and rayon tire cords, polyester tire cord has been widely used.
Prior art polyester fibers that have been used in tires have benzene lings in their molecular structure, and a rigid molecular chain. Accordingly, tire cord formed from polyester yarn has good elastic modulus and good fatigue resistance, and provides few flat spots, excellent creep resistance and excellent endurance. For these reasons, polyester tire cord has been widely used in radial tires for passenger cars.
However, in spite of the above described merits, conventional polyester tire cords do have a problem; they undergo substantial variation in their properties with temperature due, it is thought, to hysteresis effect. In particular, conventional industrial high strength polyester fibers generally exhibit substantial shrinkage when heated.
Also, when industrial polyester fibers have been incorporated into a rubber matrix of a tire, as the tires rotate during use the fiber is stretched and relaxed during each tire rotation. Further, the internal tire air pressure stresses the fiber, and tire rotation while axially loaded or stressed causes repeated stress variations, particularly on unsmooth surface.
Since more energy is consumed during the stretching of a fiber than is recovered during its relaxation, the difference of energy dissipates as heat. This is termed hysteresis or work loss. Significant temperature increases have been observed in rotating tires during use which are attributable at least in part to this fiber hysteresis effect.
The variation in properties caused by heat generation occurs due to moisture and amines contained in conventional rubber solutions used in rubber treatments for producing tire cord, and the observed variation tends to be increased when the content of carboxyl end group is high, leading to a significant lowering of strength and fatigue resistance.
In recent years, as radial tires having high performance have been widely developed and used, the demand for polyester tire cord with superior properties, especially properties superior to those obtained with nylon or rayon tire cord, has been increasing. Therefore, research into development of a polyester tire cord having improved fatigue resistance by minimizing the heat generated due to the hysteresis effect has been undertaken.
Prior art methods for improving fatigue resistance of polyester fibers have focused on a chemical method for increasing stability by reducing the content of carboxyl groups in the polyester and a method wherein highly-oriented undrawn yarn produced using a polyester with a relatively low intrinsic viscosity(IV), or produced by employing a high-speed spinning process, is drawn.
Reference directed towards increased chemical stability are Japanese patent laid-open nos. Sho. 54-132696 and 54-132697 which disclose the inhibition of deterioration due to thermal decomposition resulting from heat generation by reducing the content of carboxyl end groups. Reducing the content of carboxyl end groups via copolymerization with or melt-blending in an aliphatic polyester has a advantage in that the increased mobility of amorphous portions effects a reduction in heat generation leading to a reduction in thermal degradation which effects improved fatigue resistance. But in this method, high crystalline polyester fiber can not be obtained and the tenacity and initial elastic modulus of the material is always low. Thus, the shrinkage of the resulting fiber is increased and the product obtained is not a high quality tire cord yarn. Also, reducing the content of end groups by adding a blocking agent has a disadvantage in that the degree of polymerization is lowered and the cost is increased.
References directed to a method of increasing thermal stability are U.S. Pat. Nos. 4,101,525 and 4,195,052 which disclose an improvement in fatigue resistance by increasing the mobility of the molecular chains in the amorphous portions using high-speed spinning. In this process, the fatigue resistance is improved but the amorphous molecular chain length is irregular and long, and the relaxed molecular chains coexist so that the loss of tenacity is high. Also, the difference in properties between the inner and outer layers of fiber is effected so that the drawability decreases. The resultant variation in physical properties within these portions of the fiber is severe due to the presence of a defective microstructure.
Prior art processes for producing tire cord from yarn include, for example, Japanese patent laid-open no. Sho 61-12952 which discloses a tire cord having a tenacity of at least 7.0 g/d, an absorption peak temperature in the amorphous portions of 148.degree.-154.degree. C., a dry shrinkage of 3.3-5% which is produced by spinning a polyester having an intrinsic viscosity of 1.0, a diethylene glycol content of 1.0 mol %, a carboxyl group content of 10 eq/10.sup.6 g at a spinning speed of 2,000-2,500 m/min to obtain undrawn yarn, drawing the undrawn yarn at about 160.degree. C., thermally treating at 210.degree.-240.degree. C. and dipping the obtained yarn in a conventional rubber solution.
In addition, U.S. Pat. Nos. 4,101,525 and 4,195,052 disclose a polyester tire cord produced by a process comprising drawing highly oriented undrawn yarn prepared from a high-speed spinning process to obtain highly oriented drawn yarn, specifically multi-drawn yarn comprising 85 mol % polyethylene terephthalate having a denier per filament of 1 to 20 and a work loss at 150.degree. C. of 0.004-0.02 lb.in, and dipping the multi-drawn yarn in a rubber solution.
In the above methods, the tie molecular chains, which greatly affect dimensional stability (especially shrinkage), are oriented. This leads to residual internal stress and finally causes a lowering of the fatigue resistance of the tire cord. In most of the conventional polyester yarns for tire cord, internal stress causes a temperature rise which induces a continuous increase of thermal stress. This finally results in poor tire cord fitigue resistance because after the tire cord conversion process or dipping process comprising dipping the cord in a rubber solution and thermally treating, an internal stress remains in the tire cord.
Moreover, yarns which are highly oriented drawn yarns before undergoing the tire cord conversion process have a definite two-phase structure with both crystalline and amorphous portions. When it is dipped in a rubber solution and thermally treated, breaking of molecular chains occurs and leads to a lowering of strength.
In addition, Japanese Patent laid-open no. Sho. 54-77794 discloses a process which comprises treating polyester drawn yarn with an epoxy resin compound prior to dipping in a rubber solution but this process did not solve the above-described problems.
The present invention has been developed to solve the above described problems of the prior art. According to the present invention, the two problems of lowering of fatigue resistance due to residual internal stress by high-speed spinning and lowering of strength due to deterioration of the crystalline portions on dipping in a rubber solution can be solved based upon the points described below.
Polyester yarn having a high crystallinity undergoes a high degree of the thermal hysteresis and, accordingly, has a high thermal stress. Thus, it tends to undergo a lowering of strength, elastic modulus or strength retension efficiency due to formation of folded crystals and in particular, from unconstrained molecular chains in the amorphous portions present during recrystallization which subsquent heat treatments, such as dipping process etc., cause.
In addition, a high crystalline polyester yarn itself may exhibit microstructally good thermal stability, dimensional stability and fatigue resistance , but these properties are still insufficient because the -yarn has not any functional groups for bonding with rubber. Accordingly, the yarn is twisted to improve its fatigue resistance and is subjected to dipping process(latex treatment) to improve bonding property with the rubber. All conditions of thermal energy, shrinkage and relaxation heat treatment etc. in dipping process of the yarn determine the mechanical properties and final dimensional stability of tire cords. From such variation, the present inventors have found that a microstructural variation during a series of cord conversion processes is a important technical point in improvement of cord qualities such as dimensional stability and fatigue resistance.
The present inventors have directed their research toward improving the prior art methods for producing polyester yarn for tire cord which has excellent overall physical properties like strength, and at the same time, which has a high strength conversion efficiency and excellent dimensional stability leading to excellent fatigue resistance when used even after having been subjected to a high temperature dipping treatment as well as after having been incorporated into a rubber matrix. As a result of this research, the present invention has been achieved.
Thus, in order to improve the mechanical properties and thermal shrinkage of filamentary yarn, most of the conventional methods form filamentary yarn with an ideal structure which maintain high degree of crystallization and minimize the degree of orientation of amorphous portions, through processes taking with the thermal treatment of high temperature, and dip the filamentary yarn into a rubber solution to accomplish properties required as final tire cords.
But, the high temperature processes accompanied in such methods is causative of increasing remaining thermal stress, limitation to the high speed production of filaments and a rise in cost due to an increase in required energy. Also, thermal energy required in dipping process have to be higher than energy required in the preparation of filamentary yarn to relax thermal stress accumulated in the preparation of filamentary yarn so that the dipping speed is restricted. Also, such methods restrict microstructual variations in dipping process, and therefore, are unfavorable to accomplish the mechanical properties and dimensional stability of cords.
The present invention comprise producing a polyester tire cord having stable two-phase structure of crystalline and amorphous portions by defining a density, which represents a crystallization level, within a limited range from the preparation of yarn, maximizing birefringence index of amorphous portions to form fibrous microstructure and then recrystallization using thermal energy generated during dipping in rubber solution to rearrange the fibrous microstructure.
Amorphous portions with high degree of orientation existing in filamentary yarn is easily crystallized when they are subjected to thermal treatment during dipping. The size of such crystal in the present yarn is small than that of conventional yarn with 10% or more. Accordingly, the present filamentary yarn in a cord has a network crystal structure which crystalline portions and amorphous portions are homogeneously distributed, and therefore, have a excellent dimensional stability. In particular, the content of constrained tie molecular chains connecting between crystals is increased by minimizing formation of folded crystals during recrystallization so that high elastic modulus can be remained.
Moreover, the present inventors have discovered particular spinning and drawing process which achieve the above characteristics. Consquently, the process conditions necessary to produce an excellent polyester filamentary yam have been designed. In more detail, an undrawn yarn is produced which has highly oriented molecular chains in amorphous phase shch that crystalline diffraction is not observed clearly by x-ray, thereafter the undrawn yarn is drawn at a low draw ratio and a low temperature (below the crystallization temperature) so as to minimize the strain of molecular chains in amorphous portions induced by drawing, and then subjected to thermal treatment and relaxing at a low temperature so that no further crystallization occurs. The filamentary yarn is then dipped into a rubber solution and thermally treated at certain temperature and tension conditions enabling recrystallization to occur, thereby obtaining a final polyester tire cord.